Storage media for computers and other types of electronic devices generally come in two types: volatile memory and non-volatile memory. Volatile memory loses its contents when power is no longer being supplied to the memory, whereas non-volatile memory maintains its contents even when power is not being supplied to the memory. The most common type of volatile memory is random-access memory (RAM), which is most commonly available as and implemented as an integrated circuit (IC). The term data storage medium is used herein in a broad sense, and encompasses IC memory, as well as other types of data storage media.
By comparison, non-volatile memory has perhaps more commonly been available as and implemented as magnetic and optical media, including hard disk drives, floppy disks, compact disc read-only memories (CD-ROM's), CD re-writable (CD-RW) discs, and digital versatile discs (DVD's), among others. Historically, non-volatile memory implemented as an IC was primarily available as ROM that was not re-recordable, such as hard-wired ROM and programmable ROM (PROM). More recently, IC non-volatile memory has become available as various types of flash memory, which is more technically known as electrically erasable PROM (EEPROM).
IC-based data storage media is typically read from and, where applicable, written to, using electric signals. By comparison, magnetic and optical data storage media is typically read from, and where applicable, written to, using magnetic sensors and optical sensors, where the latter usually include some type of optical beam. However, a new type of data storage medium has been contemplated that can be implemented as an IC, but that is read from and written to using an emitted beam of electrons. A phase-changeable layer is present that can be switched between two different phases by being subjected to the emitted beam. The two phases correspond to different logical values, such as binary one and binary zero.
For reading the logical value stored in the phase-changeable layer, the medium is subjected to the emitted beam at a low intensity, whereas for writing a logical value to the phase-changeable layer, the medium is subjected to the emitted beam at a high intensity. For such reading from and writing to the re-recordable data storage medium, the spot size of the emitted beam, which is also referred to as just the size of the emitted beam, is desired to be a predetermined size. If the size of the beam varies from this desired size, the storage density may be reduced and errors may result when reading from or writing to the medium. Beam size variation can occur, for instance, because of manufacturing tolerances in the gap between the emitter and media, the divergence angle of the electrons emanating from the emitter, as well as the effectiveness of the lens to focus the emitter.